Athletic shoe soles have been made with a variety of resilient cushioning elements for cushioning a wearer's feet, such as by storing and absorbing impact energy. Known cushioning elements include bladders enclosing material that is pressurized, such as to a pressure greater than the ambient pressure surrounding the cushioning element. Typical materials include gases, viscous liquids, and gels. The cushioning properties of these known shoe soles depend upon retaining the pressurized state of the enclosed material.
A cushion element for a shoe sole would ideally provide cushioning properties that vary as a function of position. For example, a cushion providing a stiffness that is greater along a medial edge relative to a lateral edge would tend to reduce pronation compared to a cushion lacking such differential stiffness.